Scientists are interested in learning more about the key weeks after salmon enter the ocean because it seems to set the tone for the strength of the annual class.

“Much of their health and the success of their subsequent runs upstream to start the next generation are dictated by those first few weeks in the ocean,” said Geoff McMichael, the PNNL scientist who led the study, published in the journal Animal Biotelemetry.

By learning more about the first crucial weeks, better decisions may be made about when to release juvenile fish from hatcheries or to adjust plans for barging salmon downstream past dams based on ocean conditions.

If the water is cool and ocean currents and winds are bringing plankton up to the surface to provide food, juvenile salmon have a high survival rate and that’s reflected as the fish later return to the Columbia River, McMichael said.

But warm water or not much food keep them from thriving, he said. When the ocean is unusually warm, Pacific hake are more likely to come closer to the mouth of the river and feast on young salmon.

The study also found that the biggest and fastest fish in the study, steelhead, are the most likely to fall prey to predators. Although the team did not investigate the reasons for that, steelhead may be more vulnerable to fish-eating birds like terns and cormorants because they swim closer to the surface than chinook salmon, according to McMichael.

His team set out to design a study to look at assumptions about salmon mortality in the ocean based on data collected by monitoring salmon north of the mouth of the Columbia. The study’s finding that not all fish head north immediately indicates the survival of salmon in the ocean may have been underestimated, he said.

The data also showed that the length of time salmon spend in the transition zone between the mouth of the river and the ocean varies.

Steelhead are more likely to swim straight into the ocean, but the youngest chinook salmon — those less than a year old — are likely to go back and forth for a few days before remaining in the ocean.

The study, which accumulated several hundred million pieces of data about the behavior of salmon, was possible because of a fish-tagging system called the Juvenile Salmon Acoustic Telemetry System that McMichael and others developed over the last decade.

Ecologists implanted battery-powered transmitters into more than 8,000 migrating fish and released them from 140 to 245 miles upstream from the ocean, where an array of receivers was set up in a three-sided box. Detectors were dropped near the ocean floor, each about 1.5 miles apart.

The transmitters, each the size of a couple of grains of rice, emit a high-pitched beep from the fish, sending a ripple of sound through the water that the receivers can detect among other underwater sounds. The transmitters emit signals every few seconds for at least a month and can be detected by receivers up to 820 feet away.

Passive integrated transponder tags, which are less expensive and smaller, are more commonly used to track fish. But they can be read only if fish swim within a few feet of a detector, making an array in the ocean impractical.

Even with the better battery-powered transmitters, scientists know that many fish likely did not get close enough to the receivers to be detected.

But they were able to detect about 1,701 fish as they swam straight out or turned to the north or south at the ocean in a perimeter around about 100 square miles of water. Most of the fish are assumed to eventually turn to the north, although the study did not track how far they swam in other directions before heading north, McMichael said.

The research was supported by the Portland district of the U.S. Army Corps of Engineers. Other authors of the report include PNNL scientists Amanda Hanson, Ryan Harnish and Donna Trott.